AARP, an organization representing millions of older Americans, isn't on board with the House Republican plan to repeal and replace the Affordable Care Act passed out of the chamber last week.

In a letter sent to members of Congress prior to the U.S. House vote on the American Health Care Act Thursday, executive vice president and chief advocacy officer Nancy LeaMond said the legislation would be a step backward in the health care reform debate and could result in higher premiums or a loss of health care for many older Americans.

The group was critical of the amendment put forward by Michigan Congressman Fred Upton, R-St. Joseph, that would add $8 billion over five years to assist those with pre-existing conditions in states that seek waivers under the Republican health care plan as currently proposed.

"The Upton amendment would do little to reduce the massive premium increases for those with pre-existing conditions," LeaMond wrote.

The AARP has argued the American Health Care Act as proposed would do harm and increase costs for older Americans, especially for 50-64 year olds who purchase health care coverage on the individual insurance market.

In the letter, LeaMond said the AARP plans to inform members through publications, email alerts and in the media how elected officials voted on the legislation.

Upton became a critical figure in the health care debate when he when he announced he wouldn't support the bill without assurances people with preexisting conditions would be protected. Upton is a former chair of the House Energy and Commerce Committee, which plays a key role in health policy.

On Wednesday, he told reporters they would support the legislation with an $8 billion amendment added over five years to assist those with pre-existing conditions in states that seek waivers under the Republican health care plan as currently proposed.

In a statement immediately following the vote, Upton said the status quo is unacceptable and that the legislation is a first step in the right direction for ensuring quality, affordable health care.

"I told the president directly this week that I would not support this bill as written without a fix. He took the challenge and we walked him through the problems and potential solutions," Upton said in the statement. "No, this process has not been perfect. I have heard from many on both sides of this debate. I pledge to continue listening and will be a constructive partner in making our healthcare system work better for all Americans."

All nine Republican members of Michigan's Congressional delegation voted for the legislation, while all five Democrats in the delegation voted against it.

The final vote in the U.S. House was 217-213, with 20 Republicans voting against the legislation. The health care debate is now in the hands of the Senate, where leadership has indicated they plan to write their own version of a repeal and replace of Obamacare.